This invention relates to a method of determining deterioration of an oxygen concentration sensor for sensing the concentration of oxygen in exhaust gases emitted from an internal combustion engine, and more particularly to a method of this kind, which utilizes an oxygen concentration sensor having an output characteristic such that the output thereof is proportional to the concentration of oxygen in the exhaust gases.
Electronic fuel supply control for internal combustion engines generally includes air-fuel ratio feedback control wherein the concentration of oxygen contained in exhaust gases emitted from the engine is detected, and the air-fuel ratio of a mixture actually supplied to the engine (hereinafter called "the actual air-fuel ratio") is controlled to a desired air-fuel ratio in a feedback manner responsive to the detected concentration value, in order to improve emission characteristics, fuel consumption, and so on. Conventional air-fuel ratio feedback control methods include methods using a proportional output type sensor as the oxygen concentration sensor, which has such an output characteristic as to produce an output proportional to the concentration of oxygen in the exhaust gases.
However, such proportional output type sensor is required to have an output accurately corresponding to the oxygen concentration for properly controlling the air-fuel ratio. However, after a long-term use, the sensor is apt to deteriorate mainly due to deposition of oxides in the exhaust gases on electrodes of an oxygen concentration sensor element of the sensor, failing to maintain a desired output characteristic.
To cope with the above problem, a method of determining deterioration of an oxygen concentration sensor has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 60-218058 which utilizes the characteristic of output current (hereinafter referred to as "pumping current") produced from an oxygen concentration sensor element of the aforesaid type of oxygen concentration sensor that, as shown in FIG. 8, in which the pumping current is plotted with respect to voltage applied to the sensor element, the pumping current has a flat curve portion A where it assumes a constant value even when the applied voltage varies, and the inclination of a rising curve portion B decreases to shorten the flat curveportion A as the sensor element deteriorates. According to the proposed method, the determioration of the oxygen concentration sensor is determined by comparing between values Il and Ia of the pumping current respectively produced when two different voltages Vw and Va are applied to the sensor element.
However, in the above proposed method, the sensor element to which voltage is applied is the same element that has its pumping current detected. Therefore, when the pumping current flows through the sensor element, there arises a voltage drop porportional to the value of the pumping current across the sensor element due to the internal resistance thereof, so that the applied voltage is detected as the sum of the actual applied voltage and the voltage drop. Therefore, the deterioration of the sensor is judged based on the relationship between the inaccurately detected applied voltage and the pumping current value, which makes it impossible to determine the deterioration of the oxygen concentration sensor with accuracy.